Rotary-wing aircraft such as helicopters typically comprise a fuselage, a tail boom fixedly coupled to and extending from the fuselage, a pylon fixedly coupled to an end of the tail boom, an engine and transmission mounted on the fuselage, and a rotor system coupled to the engine and transmission by a rotatable mast (drive shaft).
The rotor head comprises a centrally-located hub to which the rotor blades are mechanically coupled. The rotor blades generate lift that suspends the fuselage below the rotor blades during flight. The overall lift is typically controlled by a collective control that collectively varies the pitch of the rotor blades. Directional control of the helicopter is usually achieved, in part, by a cyclic control that varies the pitch of each rotor blade on a cyclic basis so as to asymmetrically vary the overall lift.
The plane of rotation of the rotor blades, in general, must be tilted forward for the helicopter to fly in the forward direction. Helicopters are often constructed so that the plane of rotation of the rotor blades is angled in relation to the longitudinal axis of the fuselage (this angle is commonly referred to as “mast tilt”).
Mast tilt, under certain conditions, can allow the fuselage to remain level, or nearly level, in relation to the direction of flight. Operating a helicopter in this manner is desirable because the aerodynamic drag exerted on the fuselage is believed to be at or near its minimum when the fuselage is level in relation to the direction of flight. Achieving a desired airspeed under other operating conditions, i.e., at other airspeeds, weights, weight distributions, etc., may necessitate tilting the plane of rotation of the rotor blades to an extent that causes the fuselage to be angled to a substantial degree in relation to the direction of flight. Operating a helicopter with a substantial angle between the fuselage and the direction of flight can lead to excessive drag and a reduction in the maximum airspeed achievable at a given operating condition.
Rotor heads that allow the plane of rotation of the rotor blades to vary in relation to the drive shaft (and the fuselage) have been developed. These types of rotor heads, however, are typically “free-floating” systems. In other words, the relative orientation of the rotor blades and the fuselage is determined, to a large extent, by the aerodynamic forces acting on the rotor blades. Hence, these types of systems do not facilitate positive control of the angle between the rotor blades and the fuselage so as to permit the angle to be optimized to achieve minimum drag and maximum airspeed for the helicopter.
The concept of a rotor head that incorporates a universal-joint-type mechanism to pivotally couple the rotor blades to the mast has been developed. The use of a universal-joint-type mechanism in such an application can subject a helicopter to excessive vibration, however, as universal joints typically generate harmonic vibrations when used to transmit torque between two non-aligned shafts.
Flexible materials, such as elastomeric materials, have been used in rotor heads to facilitate varying the angle between the plane of rotation of the rotor blades and the fuselage. Flexible materials are generally unsuited for the high stresses that may be generated in such load-bearing applications, however. Hence, the use of such materials in this manner can adversely affect the durability, reliability, and safety of the rotor head.